Rabies Danger, Exposure Cost Reinforce Equine Vaccination Need

Rabies is a rhabdovirus that has been a human threat since antiquity. The virus is capable of infecting all warm-blooded animals with some variation in susceptibility. Rabies is considered 100% fatal to the infected host. However, in order for any disease to perpetuate itself, 100% is not in the realm of "mother nature," or the disease would kill itself out. Certain species must, therefore, act as carriers or have significantly prolonged incubation periods, allowing them to infect additional hosts.

Biting and contact with a rabid host's saliva is the mainstay of transmission, and once infected the incubation period to the onset of clinical signs varies from a few days to months. Being bitten about the head allows the virus to travel more directly to the nerves and brain as opposed to the rump, where muscle slows the infectivity and the virus encounters a longer trip to the nervous system.

Historically, horses or donkeys (equids) have not transmitted the disease to humans in the United States. Rare instances have occurred in foreign countries with the donkey or ass being the transmitting species. In this country, bats, skunks, and raccoons are the most common species found to be rabid, and although approximately 60 equine cases of rabies might occur in a given year, horse-to-human infection has not resulted.

Our domestic animals sometimes contract rabies by their predation on susceptible wildlife. Most domestic dogs are vaccinated, so human encounters with rabid dogs are relatively uncommon. Cats as predators are more agile in their hunting techniques, and are therefore less likely to be bitten.

Another form of transmission to humans is aerosol. Bat caves are considered the most likely environment for this form of the acquisition of rabies, but air is not currently considered infective wherever the environment is not closed or perpetually infested.

A rabid horse is sedated heavily and secured.

In the horse clinical signs of rabies vary. It is said that an infected rabies patient can present without any consistent rules regarding the clinical presentation. Classically, there are two forms of clinical signs, the more common "dumb" or stuporous form, and the dreaded "furious" form, in which the host is on the attack. More rare presentations have been lameness, colic, muscle stiffness, recent poor performance, incoordination (ataxia), and a host of nondescript signs that in hindsight confirm the lack of continuity to the disease until progression leads to obvious nervous system involvement.

Exposure to rabies includes any threat of contact with saliva, especially to mucous membranes or open wounds. Officials might require that animals suspect for rabies (if not euthanized or dead) be quarantined for up to six months. Confirmation of rabies requires examination of the brain for the histological presence of Negri bodies (round or oval inclusion bodies seen in the cytoplasm and sometimes in the processes of neurons of rabid animals after death) or antibody tests that adhere and illuminate the virus with fluorescent antibody. Rabies is a reportable disease and human exposure will be treated with vaccination and the use of antiserum. Prevention in humans--including those at high risk for exposure, such as veterinarian-- is by vaccination.

Selected experiences with rabies cases

A 2-year-old mule was presented recumbent (down) after an attempt at halter breaking. She was examined by clinicians at a university, who suspected trauma to the spinal cord. A total of around 80 students and faculty conducted examinations of the mule--including oral examinations--during clinical rounds. Later, radiographs showed neither skull nor neck fractures. This was followed by the mule experiencing severe seizures during the night and death in the morning. Necropsy revealed she was positive for rabies. The exposed students and faculty had to undergo vaccination and receive antiserum. The cost of treatment exceeded $15,000, but it was successful in the prevention of human cases.

A 5-month-old Standardbred colt was examined for severe depression and stuporous behavior. Two veterinarians performed a spinal fluid sampling. One veterinarian was from Ireland, a country free of rabies. The foal died the following day and tested positive for rabies. The American veterinarian had previously been vaccinated and received a booster vaccination. The Irish veterinarian was unvaccinated and underwent a vaccination series and rabies antiserum injections. About six weeks later the Irish veterinarian missed one of his antiserum treatments by leaving the country with a flight of racehorses sent to Ireland. He was reported by the authorities as having missed a treatment and was arrested by the Irish department of public health.

A 2-year-old racehorse stabled at a racetrack developed maniacal behavior consistent with the furious form of rabies. The veterinarians suspected rabies, but could not get into the stall to euthanize the horse. Word about the bizarre horse spread and more than 30 onlookers gathered to watch as the veterinarians resorted to shooting the horse from outside the stall door. Test results were positive for rabies. Physicians attempted to treat every possible exposed human with vaccination and the antiserum series. One previously vaccinated veterinarian refused the antiserum based upon having an existing antibody titer and questioned as to whether the antiserum had been tested for AIDS/HIV. It turned out that all but four people were exposed to rabies by contact with the remaining people treated, not by having true exposure (to the horse). They were treated with a serum of human origin that had not been HIV tested, but that thankfully was later batch-tested and found to be negative for the HIV virus.

A mature broodmare was presented for severe colic that resulted in surgical exploration of the abdomen. Gastrointestinal lesions were not found during the exploratory surgery. Postoperatively, the mare experienced violent seizure activity followed by euthanasia. She tested positive for rabies and all direct contact farm and clinic personnel had to undergo vaccination and treatment. The gloves and masks worn by the surgeons were not considered adequate to rule out exposure, and they were included in the full treatment program.

An unvaccinated pleasure horse presented three-legged lame and was suspected of harboring a sole abscess. The hoof was wrapped in a poultice and the horse was left in a round pen overnight, only to be found dead in the morning. The test for rabies was positive and the farm employees had to be treated.

Summary

Rabies is preventable through vaccination of horses and other domestic animals. Annual vaccination is recommended, although recent research suggests that vaccine protection might last longer in some species than others. Exposure is through direct contact, with aerosol contact limited to confined circumstances, such as being in a stall where the horse's saliva or secretions could possibly contaminate the air. Wearing masks and gloves can reduce exposure, but treatment may still be required by the public health department, especially for unvaccinated individuals. It takes about 28 days to develop immunity after vaccination. There are no completely acceptable diagnostic tests in the live animal and confirmation of rabies requires the examination of brain tissue. Necropsies in suspect cases of rabies involve removal of the brain for testing, and the remaining carcass is not examined until a negative test result is confirmed.